Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 1,821	$ 1,996
Interest cost	3,414	3,955
Expected return on plan assets	(5,580)	(4,908)
Amortization of net actuarial losses/(gains)	1,634	2,167
Amortization of prior service cost/(credit)	(62)	(70)
Net periodic benefit cost (benefit)	1,227	3,140
Estimated employer contributions in current fiscal year	12,500
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	309	312
Interest cost	214	291
Expected return on plan assets	(1,852)	(1,445)
Amortization of net actuarial losses/(gains)	(190)	0
Amortization of prior service cost/(credit)	13	(8)
Net periodic benefit cost (benefit)	$ (1,506)	$ (850)